Osteosynthesis device

ABSTRACT

Intramedullary osteosynthesis device is capable of adopting a profiled H-shape in a closed introductory state in order to be easily implantable and an X-shape in an open implanted state for good fixation in the bone.

The invention relates to the technical field of orthopaedic implants, in particular for arthrodeses and osteosyntheses.

It may be recalled that an osteosynthesis implant must serve to hold in place two (or more) parts of the same bone fractured or cut by a surgical operation (osteotomy), for the time necessary for the consolidation of this bone (typically 3 months).

It may be recalled that an arthrodesis is the blocking of a joint by surgery to fuse two bones into a single one, using an osteosynthesis device.

It may be recalled that the purpose of any osteosynthesis and particularly in the case of an arthrodesis is to obtain very good primary and also secondary stability, in order to obtain the consolidation in the best possible conditions, that is, in a position selected by the surgeon, by minimizing the problems of post-operative pain and oedemas, by shortening the consolidation time as much as possible.

To obtain this result, the stability of the osteosynthesis associated with the implant is critical. Furthermore, the implant must also provide and maintain a slight compression on the portions of bone to be fused together, thereby facilitating this consolidation.

Various technical solutions have been proposed to carry out an arthrodesis, in particular at the tips (foot, hand, wrist, ankle, etc.).

Mention can be made, for example of basic agraffes which do not provide a proper fixation during consolidation, and shape memory agraffes which serve to place the two bone portions to be consolidated under compression, thereby corresponding to the intended purpose.

However, to obtain satisfactory stability, it is necessary to place two, or even three agraffes, in different planes. This significantly increases their size, thereby limiting the applications, in particular on small bones (for example in the fingers or toes).

It is also common to use extramedullary or extra-osseous plates and screws, which also require a relatively large space and cannot be used on the terminal phalanges of the fingers (distal interphalangeal arthrodesis for example). Moreover, the medium-term stability of these systems is not always guaranteed (loosening of the mounting).

Certain types of screw can be used in intramedullary cases, but in this case, the approach path requires a pulpar approach, which may generate serious complications (sepsis, etc.) and discomfort for the patient.

Use can also be made of pins which have a smaller size. However, the stability obtained is not satisfactory (problems of migration) and it is generally necessary to remove them after consolidation. With such devices, moreover, the patient cannot immerse the finger or toe treated, because the pin generally projects outside the skin.

Intramedullary osteosynthesis systems are available for long bones (tibia, femur, humerus, etc.). For example, lockable centromedullary nails are known. Apart from the fact that the locking technique is difficult, it cannot be miniaturized for tip surgery (hand and foot).

Shape memory intramedullary devices are also available for solving part of the problematics of the arthrodesis or the osteosynthesis of the small fragments: for example, the solutions described in U.S. Pat. No. 2,846,545 or U.S. Pat. No. 2,884,406.

U.S. Pat. No. 2,846,545 describes an H device which opens in the body into an X, thanks to the use of a shape memory set around 37° C., each leg being implanted in a calibrated hole.

In practice, such a system does not allow proper introduction into the bone. This is because the preparation of 2 parallel holes in a phalange is extremely difficult due to the limited size and, above all, the parallel legs tend to open naturally during introduction and thereby exert an effect of distraction of the two fragments rather than compression.

Furthermore, the use of shape memory is very limiting due to the demands it makes on surgeons, in particular of temperature management: the implant must be impacted into the bone in their closed cold phase before they open at the temperature of the block. This requires placing the implant in a support, storing it cold, and using all possible speed for implantation.

Finally, since the legs are straight, their shape memory tends to create a local support at their tips, which does not ensure satisfactory behaviour and can damage the bone.

U.S. Pat. No. 2,884,406 teaches a system for easier introduction whether by the shape (eye) or by a support or a clamp that keeps the legs of the implant closed during introduction.

Nevertheless, these systems do not operate very dependably, because they do not define the optimal criteria allowing proper introduction into the bone and good anchorage: the anchoring zones always tend to open too early, thereby blocking introduction.

It is the object of the invention to remedy all these drawbacks simply, reliably, effectively and efficiently.

The problem that the invention proposes to solve is to define the success criteria for an intramedullary implant, easy to place and effective for generating primary and secondary stability of the osteosynthesis or arthrodesis focus thanks to its stiffness and its compression component.

Invention

The inventive implant is characterized in that it comprises two bone anchoring zones on either side of a stiff stability zone, withstanding shear forces, these two anchoring zones having a possibility of high deformation at their base (in particular by elasticity) and a design such that they can adopt a closed position (in particular thanks to a suitable clamp closed at their base) for easy introduction into a calibrated centromedullary hole (prepared with an appropriate instrument), and in that owing to this particular configuration, they offer in the bone site the possibility of obtaining the final impaction without a distraction effect on the bone and sufficient expansion to ensure proper fixation in the bone.

The anchoring zones can be deformed at their base by elasticity, superelasticity or shape memory and typically consist of branches or legs, optionally connected (olive or rugby ball shape). In the open shape, these branches have an outward positive angle at their base and are curved inwardly towards their tips, whereas when the shape is closed, the angle of the base is reversed, that is negative or turned inwardly, thereby obtaining the width at the tips (impaction side) that is smaller than their base, in order to avoid impaction in the bone and blocking of the penetration of the implant.

The invention has a particularly advantageous application, which cannot however be considered as limiting, for the preparation of arthrodeses in the phalanges of the fingers and toes, especially for the proximal and distal interphalangeal joints in the hand and foot.

The device is implantable via the dorsal path (or optionally lateral or palmar/plantar), but without pulpar approach, thereby minimizing the risks of infection and improving the patient's comfort.

To take account of the anatomical features, the anchoring zones are connected to the median zone serving for strength (in particular shear) at the osteosynthesis focus by more or less long connecting zones, and the central zone may have an angulation to adapt to the characteristics of the desired arthrodesis.

The material constituting the implant of the invention must allow a certain minimal opening of the anchoring zones once the implant is in the body. It may therefore be made from any sufficiently elastic implantable material such as stainless steel, titanium, a bioresorbable material such as polylactide acid PLLA.

Preferably, the implant of the invention is prepared from a shape memory material used for its property of superelasticity (or elasticity associated with the transformation of the austenite martensite phase under stress) which has the widest known elasticity range (up to 8% in elastic elongation equivalent in traction for an implant of Nitinol, a nickel-titanium alloy comprising about 55.5 to 56% by weight of nickel, the remainder being titanium).

It is also possible to use a material having a thermal shape memory around 37° C.

The invention is described in greater detail below in conjunction with the appended drawings in which:

FIG. 1 shows an exemplary implant of the invention in its full open position in 3 dimensions,

FIG. 2 shows the same example in its closed introduction position in its main plane,

FIG. 3 shows an exemplary implant of the invention in its full open position, in its main plane,

FIG. 4 shows an example of a bent implant according to the invention in its full open position,

FIG. 5 (5 a to 5 f) shows the implantation sequence: closed, half introduced still stressed, fully introduced on one side (free in the bone), and the same sequence on the other side.

DESCRIPTION OF THE FEATURES OF THE INVENTION

The implant is in the form of 2 anchoring zones (A1) and (A2) connected by a central zone (C) (FIG. 1) and optionally intermediate connecting zones such that in the closed position, the shape is substantially inscribed in a very elongated rectangle (FIG. 2), and in the open shape, corresponds to a wider X-shape due to the opening of the anchoring zones (FIG. 3).

The anchoring zones (A1) respectively (A2) are generally each prepared with two legs (P1), respectively (P2) having a length (L1), respectively (L2) (FIG. 3).

The cross-section of the implant is adapted to the implantation sites, but preferably flat in order to have good mechanical strength and reduced size (typically the thickness (e) is about 1 to 2 mm) (FIG. 1).

FIG. 2 shows the closed position with the various widths of the implant: (Lab) is the width of the central zone (C), (L1ab) and (L2ab) are the widths at the base of the anchoring zones respectively (A1) and (A2). These 3 widths may be equal or slightly different to adapt to the bone site. Typically, the widths are about 2 to 5 times the thickness (or 2 to 10 mm). These dimensions are adapted to the various indications of the hand and foot but are not limiting because they depend on the bone site of the operated patient.

The anchoring zones (A1), (A2) are suitable for separation by elastic effect or by shape memory effect at their base, so that the maximum width in the open position at the tips (La1) and (La2) (FIG. 3) is at least equal to the width of the base of the same anchoring zone in the closed position plus 50% minimum, or plus a minimum of 1.5 mm. This means that La1>L1ab+50% or La1>L1ab+1.5 mm and that La2>L2ab+50% or La2>L2ab+1.5 mm. This opening criterion is necessary to have sufficient fixation in the bone.

As shown in FIG. 3, the legs (P1), (P2) are substantially straight at their base (on about ⅓ to half of their length) and are then rounded inwardly at their tips (on about ⅓ to half of their length). In the open position, the straight portion of the legs (P1) respectively (P2) makes a positive outward angle (a1), respectively (a2) with the implant axis (FIG. 3), whereas in the closed position, this angle becomes the inward negative angle (b1), respectively (b2) (FIG. 2). The upper portion (towards the tip) of the legs virtually undergoes no particular deformation between the two open and closed shapes.

This particular geometric arrangement ensures that in the closed position, the legs virtually touch at the tips (FIG. 2), and that the width at the tip in the closed position (La1f) respectively (La2f) is lower than the width at the base (L1ab) respectively (L2ab), thereby allowing easy introduction without distraction of the distal bone fragment and also to obtain the opened/closed movement by a local deformation at the base of the legs, that is by leaving the distal zone free for introducing this zone into the bone.

In order to obtain both easy introduction and sufficient opening movement, the angles (a1), (a2) are preferably between +5° and +25° and the angles (b1), (b2) between 0° and −15°.

Preferably, the width of the tips of the anchoring zones in the closed position (La1f) respectively (La2f) is lower than the width of the base of the said zones (L1ab) respectively (L2ab), minus 20%: La1f<L1ab−20% and La2f<L2ab−20%.

The legs or anchoring zones are thus “articulated” at their base, and can therefore be secured in the closed position on a support or even better a clamp, positioned at an appropriate location defined in particular in the case of an elastic material (for a shape memory material, this is not absolutely necessary since the shape does not change as long as the activation temperature is not reached), this clamp not covering more than half of the length of the legs, thereby allowing introduction of at least half of the implant into its recess.

The inside tangent at the tip of the legs (P1), respectively (P2) in the open position makes an angle (β1), respectively (β2) with the longitudinal axis of the implant close to 0°, in order to have a good bone contact area along the whole length of the leg in the open position and to prevent the bone from being touched by the tips alone (FIG. 3).

In the implant site, at the body temperature, the implant can still be in the closed position, or parallel or semi-open legs so that the force exerted by the opening of the legs is transmitted to the bone and ensures proper fixation.

This “olive” arrangement of the legs, associated with an “articulation” of the base and associated with a minimal introduction of half of their length allows the completion of the impaction, once the clamp has been removed.

In order to guarantee satisfactory operation, the elasticity or memory of the piece must allow a transition from the closed shape (typically width 2 to 4 mm according to the size of the site) to an open shape with a significant movement (+1.5 to +3 mm approximately).

Similarly, the force of expansion of the legs (or swelling of the olive) must be significant: typically 1 to 3 kg for an arthrodesis of the tips (force measured at 37° C. in the blocked introduction position), without being excessive: it is important for the legs to avoid opening completely and for the bone to resist so as to have a real holding force.

The legs (P1), (P2) or fins may have a rough surface or even better notches (D) (FIG. 3) on their outer surfaces intended to be impacted in the spongy bone and to exert a good anchorage. The typical height of these notches (H1), (H2) is about 0.5 mm. The opening of the legs must be at least 1.5 times this height in order to ensure real impaction of the notches in the bone or 1.5 mm.

The legs (P1), (P2) may also have a surface covered with an osseo-integration coating such as hydroxyapatite (HAP) intended to facilitate the anchorage.

To facilitate the introduction into the bone, the tips of the legs (P1) respectively (P2) are bevelled with an inward angle to the longitudinal axis of the implant (W1) respectively (W2) (FIG. 3). This angle is typically between −20° and −40°.

By tests on fresh cadavers, and experience, an optimal level of the force was determined with a minimum allowing anchorage of the notches in the spongy bone and a maximum force to be certain to avoid damaging the implantation site.

After tests and experience, an ideal zone was found with a maximum 20% of the elastic limit of the bone measured in a blocked closed shape at 37° C., which, considering the dimensions of the implant, gives rise to maximum values of about 3 kg, and the need for a rapid lowering as soon as the anchorage is obtained, or a force divided by 2 in the semi-open position (a force of 0.5 to 1.5 kg allows good holding). In fact, if the opening force is higher than about 3 kg, introduction into the bone becomes much more difficult, or even impossible above 4 kg. Finally, in order to guarantee a damage-free site, it is necessary for the force to become negligible for a virtually complete opening. These values are indicative and depend on the arthrodesis site and the bone quality.

In one version of the invention, the notches (D1) respectively (D2) on the outside of the legs (P1) respectively (P2) allow the positioning of a clamp and introduction at the base of the legs (P1), (P2) (FIG. 3). These notches are symmetrical by pairs of legs and their spacing (d) is the same on the legs (P1) and on the legs (P2).

The central zone (C) must have a minimum length (Lc) equal to the length (d) between the notches (D1) respectively (D2) so that even in case of movement of the implant during final impaction, this zone (C) remains in the arthrodesis focus and performs its resistance function.

In one version of the invention, an orifice (Or) is provided in this central zone for positioning a holding pin to prevent migration of the implant at the time of final impaction.

As shown in FIG. 4, this central zone may be angulated by an angle (Ag) defined between the 2 main planes formed by the legs (P1) on the one hand and (P2) on the other hand to adapt to the surgical requirements for adjusting the position of the arthrodesis. In most cases, the angle (Ag) is fixed between 0° (typically flat position for an index) and 30° (typically for a little finger).

As an example, an operating technique of implantation of the inventive device for the case of an elastic or superelastic implant is described as follows as shown in FIG. 5:

-   -   Approach by dorsal path     -   Resection of cartilages and osteophytes     -   Centromedullary perforation using an appropriate instrument to         make a calibrated rectangular hole having a width substantially         (L1ab) or (L2ab) and thickness substantially e (suitable rasp)     -   Closure of the clamp side P1 (FIG. 5 a)     -   Implant introduction side P1 to minimum half (FIG. 5 b)     -   Clamp removal     -   Complete introduction side P1 (FIG. 5 c)     -   Closure of the clamp side P2 (FIG. 5 d)     -   Placement of the bone side P2 on the implant side P2 to about         half (FIG. 5 e)     -   Removal of the clamp     -   Manual impaction of the bone side P2 on the bone P1 (FIG. 5 f)

Detailed Description of the Features of the Invention and Exemplary Embodiments

In a particular embodiment, intended for a distal interphalangeal arthrodesis (hand), the implant is prepared from a superelastic Nitinol alloy (nickel-titanium in the weight proportion 55.8% nickel and 44.2% titanium).

The cross-section of the central zone (C) is Lab×e=2.8×1.2 mm and the legs are asymmetrical to adapt better to the shapes of the bone, minimize the implanted metal section and allow sufficient expansion for good anchorage. The length of the legs is L2=6.5 mm distal side (P2) and L1=9 mm proximal side (P1).

The length of the central zone (C) is 3 mm, allowing a slight offset during closure, without affecting the shear strength.

To adapt to the surgeon's choice, this central zone may be bent (typically flat or 15° or 25°).

In the closed position, the width of the proximal base L1ab is 3.8 mm and of the distal base L2ab is 3.0 mm.

The opening of the legs (P1) respectively (P2) is 2.5 mm respectively 2.2 mm, that is La1 is 6.3 mm and La2 is 5.2 mm.

In the open position the angle at the base of the legs is a1=10° and a2=22°.

The straight portion is about 45% of the total length.

The curvature of the distal tip of the legs is calculated so that the angle of the tangent at the tip is β1=−5° and β2=−3°.

In the closed position, the angle at the base of the legs is b1=−4°, b2=−2°.

And the width at tip is La1f=2.5 mm and La2f=2.1 mm.

In one embodiment of the invention, the 0.5 mm high notches are distributed on the legs (1 notch at approximately 0.8 mm intervals).

The angle of incidence of the tip of the legs (including notches) is w1=33° and w2=24°, allowing easy introduction without the distraction effect between the two bone pieces to be osteosynthesized.

The rounded design of the anchoring zones serves to obtain a maximized contact area over the entire length in the open shape, with an impaction effect in the spongy bone, and hence a spongy packing effect.

In another example, more appropriate for arthrodesis of the thumb, the dimensions are rather the following:

Closed widths: L1ab=6.5 mm, L2ab=5 mm, with an opening of 3 to 4 mm approximately to obtain: La1=11 mm and La2=8 mm and L1=13 mm and L2=9 mm. 

1. Intramedullary osteosynthesis and arthrodesis device for orthopaedic surgery, in particular of the hand and the foot, having a substantially flat main cross-section, with an H-shape in a closed introductory state comprising 2 anchoring zones, each anchoring zone comprising a set of substantially parallel legs extending from a base of a respective anchoring zone along a length to a respective tip, and the 2 anchoring zones being connected by a central zone, in the closed introductory state, the device being substantially inscribed in an elongated rectangle, and in an open implanted state, the device having an X-shape with a larger width, wherein: in the open state, each of the legs of a set makes at the base, on a 1^(st) third of the length, an outward angle of between 5° and 25° to an implant axis, and is inwardly curved on a last third of the length towards the respective tip, so that an inside tangent at the tip makes an angle of close to 0° to a longitudinal axis of a neighbouring implant, in the closed state, each of the legs of the set makes at the base, on the 1^(st) third of the length, an inward angle of between =0° and −15°, without any significant change along the last third of the length, so that an outside width at tips of a set, in the closed state and on about half of the length of the legs on a tip side, is shorter than a closed introduction width of the base of the respective anchoring zone, and transition from the closed state to the open state occurs by deformation of the base of the anchoring zone.
 2. Device according to claim 1, wherein the width at the tips of a set in the open state is at least equal to the closed introduction width plus 50%, or to the closed introduction width +1.5 mm.
 3. Device according to claim 1, wherein the outside width at the tips of a set in the closed state is lower than the closed introduction width of the base.
 4. Device according to claim 1, wherein the tips of a set in an introduction zone are inwardly bevelled to facilitate their introduction, by presenting an inward angle of between −20 and −40°, in the open state to the implant axis.
 5. Device according to claim 1, wherein the 2 anchoring zones have notches on an outer surface of the legs or a rough surface for easier anchoring.
 6. Device according to claim 1, wherein the 2 anchoring zones are connected by a stiff central zone suitable for withstanding shear stresses at a focus of an arthrodesis.
 7. Device according to claim 1, wherein the central zone has an angulation between the 2 anchoring zones for adjusting bone position of an arthrodesis between 0 and 30°.
 8. Device according to claim 1, wherein the device is made from an implantable material, comprising Nitinol (shape memory or superelastic nickel-titanium), elastic titanium or a resorbable material, so that the transition from the closed introduction state to the open state occurs by elasticity, superelasticity or shape memory.
 9. Device according to claim 1, wherein a zone or recess is provided in a lower half of an anchoring zone to position a suitable instrument for closure and fixation of the anchoring zone in the closed state.
 10. Device according to claim 9, wherein length of the central zone is at least equal to a length of the recess.
 11. Device according to claim 1, wherein an orifice is provided in the central zone perpendicular to a major surface of the device, for positioning an instrument or a holding pin.
 12. Device according to claim 1, wherein the central zone is H-shaped.
 13. Device according to claim 8, wherein the resorbable material comprises polylactic acid PLLA. 